The present invention relates to the use of surfactants for improving the compatibility of inorganic pigments such as titanium dioxide in aqueous coating compositions such as paint and paper coating compositions. In particular, the present invention related to the use of certain alkoxylated phosphorous-based surfactants for improving the compatibility of inorganic pigments and latex binders in aqueous paint and paper coating compositions.
Because of its unique ability to provide maximum light scattering with virtually no light absorption, titanium dioxide (TiO2) is widely used as a pigment to achieve high opacity (or hiding power) and brightness in white and colored paints, and in other coating compositions. Historically, TiO2 was produced and used as a dry powder. However, there are a number of problems associated with using TiO2 powder in the manufacture of coating compositions. In particular, the use of TiO2 powder can result in dusting and spillage, both of which not only result in pigment loss but also in an unhealthy work environment. Furthermore, the handling of the standard 25-kg bags of TiO2 powder can lead to extensive exposure to forklift and hand truck hazards, back-related injuries, and other job-related injury risks.
In addition to these problems, TiO2 powders typically must be ground to be used in coating compositions such as paint compositions. This grinding step is highly power intensive. Moreover, the grinding step can be very time-consuming thereby decreasing the rate at which the TiO2 powders can be used.
As an alternative to TiO2 powders, TiO2 slurries were first introduced to the paper industry in the late 1960""s and to the paint industry in the early 1970""s. TiO2 slurries are usually high solids content, e.g., from 60 to 80% by weight, aqueous dispersions of TiO2 that are stable over a long period of time.
To minimize the settling and aggregation of TiO2 particles in slurries, dispersants are often used in TiO2 slurries. The conventional dispersants used in TiO2 slurries include organic polyelectrolytes, 2-amino-2-methyl-1-propanol, triethanolamine, and sodium salts of polymeric carboxylic acids. In addition to dispersants, biocides are also used in TiO2 slurries to preserve the slurries.
The introduction of TiO2 slurries overcame many of the problems associated with the use of TiO2 powders and particularly the power intensive and time-consuming step of grinding the TiO2 powders. Therefore, the use of a TiO2 slurry results in a significant increase in productivity over the use of TiO2 powders. Nevertheless, despite the significant advantages of using TiO2 slurries in aqueous coating compositions such as paper and latex paint compositions, performance deficiencies do exist when using TiO2 slurries compared with TiO2 powders. Specifically, it is known, for example, that TiO2 slurries are often incompatible with the latex binder used in the aqueous coating composition. This problem manifests itself in the form of TiO2 particle clustering in the paint, which reduces the light scattering efficiency of the pigment, or in the form of lower gloss due to the presence of an incompatible dispersant in the slurry.
Therefore, there is a need in the art to improve the compatibility of TiO2 slurries and latex binders to limit the clustering of TiO2 particles and the reduction of gloss in aqueous coating compositions. Moreover, there is a need in the art to improve the compatibility of other inorganic pigments in aqueous compositions.
It has been found that the use of certain polyoxyalkylene phosphate surfactants in aqueous coating compositions limits the clustering of TiO2 particles in these coating compositions, particularly TiO2 particles added to the compositions in slurry form. In particular, the aqueous coating composition of the invention includes at least one polyoxyalkylene phosphate surfactant having the following structure: 
wherein m is 1 or 2, n is an integer from 1 to 100, R1 is C1-C5 alkyl, Oxe2x80x94R2 is an alkylphenol residue wherein R2 has the structure C6H4xe2x80x94CpH2p+1 or Oxe2x80x94R2 is a linear or branched alkyl alcohol residue wherein R2 has the structure CpH2p+1, and p is an integer from 1 to 30. The aqueous composition further includes at least one latex polymer, at least one inorganic pigment and water.
In a preferred embodiment of the invention, the surfactant includes a polyoxyethylene phosphate surfactant, i.e., R1 is ethyl. Moreover, preferably n is from 2 to 30 and more preferably from 3 to 15. For R2, p is preferably an integer from 8 to 18 and, in a particularly preferred embodiment, R2 is nonylphenyl or tridecyl. The at least one inorganic pigment is preferably selected from the group consisting of TiO2 and CaCO3. The at least one latex polymer is preferably selected from the group consisting of pure acrylics, styrene acrylics, vinyl acrylics and acrylated ethylene vinyl acetate copolymers and more preferably includes a pure acrylic such as a butyl acrylate/methyl methacrylate copolymer. The surfactant discussed above can be incorporated in the latex by using it to produce the latex or it can be added separate from the latex to the aqueous coating composition. The aqueous coating composition can further include at least one anti-freezing agent and one or more additives selected from the group consisting of plasticizers, drying retarders, dispersants, surfactants or wetting agents, rheology modifiers, defoamers, thickeners, coalescing agents, biocides, mildewcides, colorants, waxes, perfumes and co-solvents.
In one preferred embodiment of the invention, the aqueous coating composition is an acrylic latex paint composition, comprising at least one polyoxyalkylene phosphate surfactant as described above; at least one acrylic latex polymer derived from at least one acrylic monomer selected from the group consisting of acrylic acid, acrylic acid esters, methacrylic acids, and methacrylic acid esters; titanium dioxide and water. For example, the at least one acrylic latex polymer can be selected from the group consisting of pure acrylics, styrene acrylics, vinyl acrylics and acrylated ethylene vinyl acetate copolymers and is more preferably a pure acrylic.
The present invention further includes a method of preparing an aqueous coating composition. Specifically, at least one polyoxyalkylene phosphate surfactant as described above, at least one latex polymer, and at least one inorganic pigment are mixed to produce the aqueous coating composition. Preferably, the at least one inorganic pigment is in slurry form. The at least one inorganic pigment is preferably selected from the group consisting of TiO2 and CaCO3. The at least one latex polymer is preferably selected from the group consisting of pure acrylics, styrene acrylics, vinyl acrylics and acrylated ethylene vinyl acetate copolymers, and more preferably includes a pure acrylic. At least one anti-freezing agent and at least one additive selected from the group consisting of plasticizers, drying retarders, dispersants, surfactants or wetting agents, rheology modifiers, defoamers, thickeners, coalescing agents, biocides, mildewcides, colorants, waxes, perfumes and co-solvents, can also be mixed in the composition.
The method can further include the step of preparing the polymer latex binder using emulsion polymerization by feeding monomers to a reactor in the presence of at least one initiator and the at least one surfactant described above and polymerizing the monomers to produce the latex binder. The resulting latex binder can then be mixed with the at least one inorganic pigment to produce the aqueous coating composition. The step of preparing the polymer latex binder can include the steps of preparing an initiator solution comprising the initiator, preparing a monomer pre-emulsion comprising monomers and the surfactant, adding the initiator solution to a reactor and adding the monomer pre-emulsion to the reactor. Preferably, at least a portion of the initiator solution is added to the reactor prior to adding the monomer pre-emulsion. In addition, a seed latex is preferably added to the reactor prior to adding the initiator and adding the monomer pre-emulsion. Once the polymer latex binder is prepared, the latex binder is preferably chemically stripped by adding a peroxide and a reducing agent to the latex binder thereby decreasing the residual monomer content of the latex binder. The monomers fed to a reactor to prepare the polymer latex binder preferably include at least one acrylic monomer selected from the group consisting of acrylic acid, acrylic acid esters, methacrylic acids, and methacrylic acid esters. In addition, the monomers can include one or more monomers selected from the group consisting of styrene, xcex1-methyl styrene, vinyl chloride, acrylonitrile, methacrylonitrile, ureido methacrylate, vinyl acetate, itaconic acid, crotonic acid, maleic acid, fumaric acid, ethylene, and C4-C8 conjugated dienes such as 1,3-butadiene, isoprene or chloroprene. Preferably, the monomers include one or more monomers selected from the group consisting of n-butyl acrylate, methyl methacrylate, styrene and 2-ethylhexyl acrylate.
By adding the surfactants discussed above, there is little or no clustering of the TiO2 particles or other inorganic pigment particles in the aqueous coating compositions of the invention as evidenced, e.g., by cryogenic scanning electron micrograph (SEM) analysis. Therefore, the aqueous coating composition maintains better dispersion of the TiO2 particles or other inorganic pigment particles than conventional compositions that do not use the surfactants of the invention. Semi gloss paint formulations prepared in accordance with the invention also have excellent high shear viscosities, low shear viscosities, gloss and hiding power and show improvements in these properties over conventional formulations.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying figures, which describe both the preferred and alternative embodiments of the present invention.